Improve and extend .Position handling in Goldmark render hooks

Improve and extend .Position handling in Goldmark render hooks

Goldmark v1.8 reports source positions in render hooks.
Use this to build a source map that translates Goldmark positions back to
the original content source.

Now all render hooks' context object implenent the `BaseContext` interface:

```go
type BaseContext interface {
	Position() Position
	Page() any
	PageInner() any
	Ordinal() int
}
```

Closes #14663

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>